For example, in a clock signal used in celestial observation or the like in the field of radio astronomy, only a slight fluctuation in its frequency results in a serious adverse effect on an observation result.
In such field, therefore, an atomic oscillator having an extremely high frequency stability of 10−13 or more per day (for example, 10−4 Hz or less per day in a signal of frequency of 1 GHz) is used as clock signal source.
To design and manufacture a clock signal source having such high frequency stability, it is necessary to measure the frequency stability of the signal source very precisely.
Generally, methods for measuring stability of signal frequency are known to include a method for measuring directly the frequency fluctuations of the signal to be measured by a frequency counter, and a method of measuring indirectly the frequency fluctuations of the signal to be measured by measuring the cumulative value of the phase displacement with respect to the reference signal.
This method for measuring the cumulative value of the phase displacement with respect to the reference signal is specifically described in “Elementary Digital Clock Technology” written by Masaya Kihara and Sadayasu Ono, 2001, published by Ohm-Sha (ISBN 4274035549), pp. 179 to 184.
That is, when measuring the cumulative value of phase displacement with respect to the reference signal, as shown in this publication, it is general to employ the beat system using the dual mixer and time interval counter.
In this beat system, while sharing the local oscillator, the reference signal Sr and object signal to be measured Sx are transformed into beat signal by the dual mixer, and the phase difference between the beat signals is to be measured by the time interval counter.
When detecting the phase displacement with respect to the reference signal, it is generally difficult to detect directly the phase displacement of the signals with high frequency.
Accordingly, as in this beat system, by preparing the reference signal Sr as the reference of measurement, equal in nominal frequency f to the object signal to be measured Sx and equal or higher in the frequency stability, the displacement of phase of the object signal to be measured Sx can be detected indirectly.
In such beat system, therefore, there is a problem that it is difficult to correspond to an arbitrary nominal frequency due to various reasons.
To solve this problem, a frequency stability measuring system is developed by a quadrature detection method applicable to an arbitrary nominal frequency.
That is, as shown in FIG. 11, in this frequency stability measuring system 10 by quadrature detection method, first, reference signal Sr and object signal to be measured Sx are transformed into reference signals Sr′, Sx′ in low frequency band by a frequency converter 1.
The reference signal Sr′ and object signal to be measured Sx′ transformed in the frequency converter 1 are put into a quadrature detector 2.
In the quadrature detector 2, an in-phase signal I representing an in-phase component between the entered two signals Sr′, Sx′, and a quadrature signal Q representing a quadrature component are determined, and put into a phase detector 3.
This phase detector 3 detects the variance Δφ of the phase determined by the entered in-phase signal I and quadrature signal Q, and accumulates the values of the variance Δφ, and issues a cumulative value Φ of the phase displacement.
The cumulative value Φ of the phase displacement expresses the frequency fluctuation of the object signal to be measured Sx from the reference signal, and therefore from this cumulative value Φ, it is possible to evaluate indirectly the frequency stability of the object signal to be measured Sx.
Thus, the frequency stability measuring system 10 by this quadrature detection method is applicable to any arbitrary nominal frequency, but the phase detector 3 used herein involves the following problems.
This phase detector 3 is configured specifically as shown in FIG. 12.
The phase detector 3 delays the complex number (I′+jQ′) composed of in-phase signal I′ and quadrature signal Q′ entered at the previous clock timing by one clock period by a delay unit 4, and a conjugate complex number (I′−jQ′) is determined by a conjugate converter 5.
This conjugate complex number (I′−jQ′), and a complex number (I+jQ) composed of in-phase signal I and quadrature signal Q entered at the next clock timing are multiplied at a next stage in a multiplication processor 6, and thereby II′+QQ′−j(IQ′−I′Q) is determined.
This multiplication processor 6 is actually composed of four multipliers for determining (II′), (QQ′), (IQ′), and (I′Q), and two adders for adding (or subtracting) them.
The result of multiplication process is expressed as AejΔφ by using the base e of natural logarithm, phase displacement Δφ, and positive number A, and is hence transformed into logA+jΔφ by logarithmic transformation process at a next stage in a logarithmic converter 7.
Based on the calculation result in this logarithmic converter 7, the phase displacement Δφ of the imaginary part is extracted at a next stage by an imaginary part extractor 8.
The phase displacement Δφ extracted by the imaginary part extractor 8 is accumulated at a next stage by an accumulator 9.
That is, the accumulator 9 determines the cumulative value Φ corresponding to the frequency fluctuations of the object signal to be measured Sx with respect to the reference signal Sr.
In the conventional phase detector 3, however, as mentioned above, four multipliers and two adders are needed for complex multiplication process of (I+jQ) and (I′−jQ′), and the configuration is complicated, and it takes much time in arithmetic process, and high speed operation is not realized.